This invention relates to a method of welding together two metal components to form a large structure which may be suitable for use as an aerospace component and to a large structure so created. In this context, an aerospace component means one that forms part of a fixed-wing aircraft, a helicopter, a missile, a satellite, a space structure or the like. The invention relates particularly, but not exclusively, to a method of welding together two large metal components to form a part or a whole of a spar for a wing of an aircraft and to a spar so created.
In many applications it is necessary to weld together two large metal components. In many applications, it is also useful if the two components have different properties, thereby creating a structure having certain properties in one section and different properties in another section. For example, a spar in the wing of an aircraft is subject to forces in its upper section different from those in its lower section and it is therefore advantageous if the two sections are constructed from metals having different properties.
Large aircraft wings are assembled from many components by joining them together with fasteners (examples of which are bolts and rivets). Each wing typically has two main spars, a forward spar and a rear spar, both running the length of the wing. Some large wings also have a centre spar. Each spar has a length, measured in a direction from the joint of the wing and the fuselage to the wing tip, and a height, measured in a direction from the upper surface to the lower surface of the wing (typically being a maximum of the order of 1 to 2 meters in a large transport aircraft, but decreasing steadily along the length of the wing to the wing tip). The spar is relatively narrow along most of its height but the upper and lower portions are of a greater width (typically no more than 200 mm) so that the overall cross-sectional shape of the spar is usually substantially that of a ‘C’, although other shapes such as an ‘I’ are possible. The spar is strengthened at regular intervals along its length by stiffeners which are substantially the same height as the spar but have a width greater than the width of the spar away from its top and bottom portions. The stiffeners take the form of plates located transverse to the spar length.
A spar can be constructed from many individual components which are riveted and bolted together (known as a fabricated spar) or from one piece (from plate material, an extrusion or a forged billet) which is then machined (known as an integrally-machined spar). Fabricated spars can be formed from metal components having different properties, which are bolted together. However, fabricated spars are labour-intensive to assemble, requiring the drilling of many holes and the setting of rivets and bolts. They are also expensive because the assembly process requires sophisticated tooling, and they are heavier because of the material overlap and the rivets and bolts. They are also at greater risk of fatigue damage due to the large number of fastener holes. The advantages of integrally-machined spars are that material overlap is minimised and bolts and rivets are not required (thereby reducing the overall weight) and sophisticated tooling is not needed for assembly. However, such spars must be machined from one piece of material, which material must be chosen as a compromise between the requirements at the top of the spar (mainly compressive loads) and the requirements at the bottom of the spar (mainly tensile loads).
An alternative possibility we have considered is that the spar could be formed from two metal components, the metals having different properties, which have been welded together. In this case, the spar could be machined before or after welding. Welding of aerospace structures has to be carried out to the highest standards, and only certain high-quality processes are acceptable. One method that is particularly suitable for welding together metals having different properties is friction stir welding. Friction stir welding is a well-known and useful welding process but is, unfortunately, limited by the depth of welding that can be achieved. Currently the maximum depth of material which can be reliably welded by friction stir welding is about 6 to 10 mm, which makes it apparently unsuitable for welding components of large structures such as spars.